Optical communications is entering an era of coherent detection. The 40 Gbps polarization-multiplexed binary (differential) phase shift keying (PM-(D) BPSK) and the 40 Gbps and 100 Gbps polarization-multiplexed quadrature (differential) phase shift keying (PM-(RZ) (D)QPSK) technologies are being deployed commercially. The coherent communication technology will be indispensable for future 200 Gbps, 400 Gbps and even 1 Tbps transmission systems. Presently, the 40 Gbps and 100 Gbps coherent systems are still in an initial deployment stage, and more and more coherent systems will be deployed in the dense wavelength division multiplexing (DWDM) networks. For the network operation and maintenance, the operator needs to monitor the in-service performance parameters of the coherent systems, for example, the optical signal-to-noise ratio (OSNR), the dispersion, the polarization mode dispersion and the polarization dependent loss, during the real-time transmission.
The in-band OSNR is one of the important performance parameters of the coherent systems, and the in-band OSNR in-service measurement is the main issue to be solved for the deployment, operation and maintenance of the current 100 Gbps PM-QPSK and the future beyond-100 Gbps (200 Gbps, 400 Gbps and 1 Tbps) coherent polarization-multiplexed systems. A method for measuring the OSNR in the prior art is an optical spectrum analyzer (OSA)-based linear interpolation noise estimation method, in which it is assumed that no spectrum overlap exists between neighboring DWDM channels and the power in the middle between the adjacent channels is regarded as the ASE noise power. By measuring the noise power of the wavelength at the middle of two neighboring channels and by performing a linear interpolation, the ASE noise power of the channel to be measured is determined. The signal power is obtained by subtracting the noise power from the power level measured at the center wavelength of the channel; thereby the in-band OSNR may be calculated. Another method for measuring the in-band OSNR is an OSA-based signal ON-OFF method, in which the measurement of the signal power is the same as in the foregoing linear interpolation method, however, during the measurement of the noise power, the signal laser is switched off, and thus the measured optical power at the center wavelength of the channel on the OSA is the ASE noise power. Since the EDFA ASE noise level will change as the signal is on and off, the measured noise power is over-estimated. Such an error may be corrected by further calibration. Further, another method for measuring the in-band OSNR is the polarization extinction/nulling method, in which the method utilizes the properties that the signal is polarized and the ASE noise is non-polarized, and employs a polarizing beam splitter PBS to split a signal into two orthogonal polarization components and finally to use OSAs to measure their spectrum respectively. By adjusting a polarization controller to align the polarization state of the input signal with one of the main polarization axis (e.g. slow axis) of the PBS, in this case the signal in the other polarization direction will be totally eliminated and only the noise remains. Then the powers at the center wavelength of the channel in the two polarization directions are measured, respectively, where the optical power level measured in the polarization direction where the signal is extinguished is a half of the ASE noise power, and the optical power level measured in the other polarization direction is the sum of the half of the ASE noise power and the power of signal. Thereby, the power of the signal and the ASE noise may be calculated, accordingly.
Those methods in the prior art have the following issues:
1) In the non-coherent 40 Gbps and coherent 100 Gbps systems, the signal bandwidth exceeds the channel grid (i.e. 50 GHz) of dense wavelength division multiplexing system defined by International Telecommunications Union (ITU). The spectra of adjacent DWDM channels overlap with each other, thus the ASE noise in the middle between neighboring DWDM channels is shielded by the signal. Therefore, the actual ASE noise power cannot be measured accurately by the traditional OSA based noise interpolation method.
2) Although such an issue may be solved by the signal ON-OFF method, it needs to interrupt the service during the measurement, and thus an in-service measurement is impossible.
3) The polarization extinction method can solve the above issues, but the coherent 100 Gbps and beyond-100 Gbps polarization-multiplexed signals, for example, PM-QPSK signals, has two orthogonal polarization states, thus the signal and the noise cannot be separated by the polarization extinction method, thereby the method is no longer applicable.
4) Although a transmission line-card has integrated a function of real-time performance monitoring, it can only measure the end-to-end channel quality and performance parameters, and there is no way to measure the channel parameters of a transmission link between terminals. Therefore, for the 100 Gbps and beyond-100 Gbps coherent polarization-multiplexed systems, an effective method is required for measuring the parameters of the in-band OSNR in-service.